Richard C. Chu

Review of cooling technologies for computer products

This paper provides a broad review of the cooling technologies for computer products from desktop computers to large servers. For many years cooling technology has played a key role in enabling and facilitating the packaging and performance improvements in each new generation of computers. The role of internal and external thermal resistance in module level cooling is discussed in terms of heat removal from chips and module and examples are cited. The use of air-cooled heat sinks and liquid-cooled cold plates to improve module cooling is addressed. Immersion cooling as a scheme to accommodate high heat flux at the chip level is also discussed. Cooling at the system level is discussed in terms of air, hybrid, liquid, and refrigeration-cooled systems. The growing problem of data center thermal management is also considered. The paper concludes with a discussion of future challenges related to computer cooling technology.

Maintaining Datacom Rack Inlet Air Temperatures With Water Cooled Heat Exchanger

The heat dissipated by electronic equipment continues to increase at a alarming rate. This has occurred for products covering a wide range of applications. Manufacturers of this equipment require that the equipment be maintained within an environmental envelope in order to guarantee proper operation. Achievement of these environmental conditions are becoming increasingly difficult given the increases in rack heat loads and the desire for customers of such equipment to cluster racks in a small region for increased performance. And with the increased heat load of the racks and correspondingly increased air flowrate the chilled air flow supplied either through data center raised floor perforated tiles or diffusers for non raised floors is not sufficient to match the air flow required by the datacom racks. In this case some of the hot air exhausting the rear of a rack can return to the front of the rack and be ingested into the air intake thereby reducing the reliability of the electronic equipment. This paper describes a method to reduce the effect of the hot air recirculation with a water cooled heat exchanger attached to the rear door of the rack. This heat exchanger removes a large portion of the heat from the rack as well as significantly lowering the air temperature exhausting the rear of the rack. This paper describes the hardware and presents the test results showing that a large portion of the heat is removed from the rack and the temperature exhausting the rear of the rack is significantly reduced. Finally the effectiveness of the solution is shown in modeling of this water cooled solution in a data center application.Copyright

Jet impingement boiling of a dielectric coolant in narrow gaps

An experimental investigation into the effect of the chip-to-orifice gap was conducted for jet impingement boiling of FC-72 at the back surface of a 6.5 mm square thermal chip on a 28 mm square ceramic substrate. Four different types of jets were used, all of them employing a single 0.50 mm diameter orifice. A fixed position jet impingement pin with the orifice centered on a flat face measuring 7.62 mm on an edge was used as a base case. This jet supported a heat flux of about 120 W/cm/sup 2/ at a reasonable flowrate. Variable position jet impingement pistons were shown to perform as well as, but no better than the base case. Additional tests were conducted to investigate performance with variation in the chip-to-orifice gap. Thermal performance was found to be insensitive to the gap at large spacing, but below some specific gap it degraded with decreasing gap. A sudden jump in the chip temperature was discovered to occur at a specific gap. This gap length was found to vary with jet flowrate.<<ETX>>

An Assessment of Module Cooling Enhancement With Thermoelectric Coolers

The trend towards increasing heat flux at the chip and module level in computers is continuing. This trend coupled with the desire to increase performance by reducing chip operating temperatures presents a further challenge to thermal engineers. This paper will provide an assessment of the potential for module cooling enhancement with thermoelectric coolers. A brief background discussion of thermo-electric cooling is provided citing some of the early history of thermoelectrics as well as more recent developments from the literature. An example analyzing cooling enhancement of a multi-chip module package with a thermoelectric cooler is discussed. The analysis utilizes closed form equations incorporating both thermoelectric cooler parameters and package level thermal resistances to relate allowable module power to chip temperature. Comparisons are made of allowable module power with and without thermoelectric coolers based upon either air or water module level cooling. These results show that conventional thermoelectric coolers are inadequate to meet the requirements. Consideration is then given to improvements in allowable module power that might be obtained through increases in the thermoelectric figure of merit ZT or miniaturization of the thermoelectric elements.Copyright

Experimental Investigation of the Heat Transfer Performance of Arrays of Round Jets With Sharp-Edged Orifices and Peripheral Effluent: Convective Behavior of Water on a Heated Silicon Surface

In the present work, deionized water is impinged onto a heated silicon surface using square arrays of round jets. Various numbers of jets and jet diameters are used over a heated area of constant size with the orifice plate height above the heater held constant. In these experiments, the jet orifices are sharp-edged and the fluid exhaust direction is parallel to the heated surface and leaves the chip periphery through a manifold. The resulting temperature and flow data are presented in physical units as well as in groups of dimensionless parameters. A correlation is presented to reasonably predict the experimental results of this study. The techniques used for data reduction and for experimentation, including the construction of the test module, are given in detail, including a numerical conduction simulation based data reduction technique and uncertainty analysis. The results shown include flow rates ranging from 6.1 cc/s to 63.18 cc/s resulting in Reynolds numbers based on orifice diameter ranging from 141 to 6670. Jet diameters investigated in this study range from 377 μm to 1.01 mm, in square arrays of 16 to 324 orifices on an area of 18.52 mm × 18.59 mm. The resulting maximum spatially averaged effective heat transfer coefficient achieved is 7.94 W/cm2 K, and the maximum spatially averaged Nusselt number based on jet diameter is 79.4.© 2005 ASME

Recent Developments in Thermal Technology for Electronics Packaging

For more than 50 years, the development of thermal technology for electronics packaging has been an important arena for the application of advanced heat transfer techniques. This has been especially true in the evolution of electronics packaging for digital computers. Since the development of the first electronic digital computers in the 1940s, the removal of heat has played a major role in ensuring their reliable operation. Early digital computers such as ENIAC (Electronic Numerical Integrator and Computer) used vacuum tubes as the basic logic element building blocks [1,2]. These physically massive machines were cooled by forced air and by today’s standards were unreliable. The invention of the transistor by Bardeen, Brattain, and Shockley at Bell Laboratories in 1947 foreshadowed the development of generations of computers yet to come. As a replacement for vacuum tubes, the miniature transistor generated less heat, was much more reliable, and promised lower manufacturing costs. At the time it was even thought that the use of transistors would greatly reduce, if not totally eliminate, cooling concerns. This thought was short-lived as engineers sought to improve computer speed and storage capacity by packaging more and more transistors, first on printed circuit boards (PCBs), and then on ceramic substrates.

Thermal Management of Flip Chip Packages

Generally speaking, the electrical energy that is supplied to electronic devices is ultimately transformed into and dissipated as heat. This generation of heat is accompanied by a temperature rise at the heat source followed by the transport of heat to regions of lower temperature within and outside the electronics module or package. Within the package transport of heat occurs via a process of thermal conduction in the solid material making up the package. As the heat reaches the external surfaces of the package it is usually transferred to a cooling fluid (e.g., air) via a thermal convection process. In the case of lower power components thermal radiation may also play a role in transferring heat to the surrounding environment. The temperatures within the electronics package will continue to rise until the rate of heat removal from the package is equal to the rate of heat generation. It is worthwhile to note that, even if purposeful active measures were not taken to cool the package, the laws of nature or physics would prevail and limit the temperature rise. However, in most instances, the resulting temperatures would be too high. As shown in Fig. 9.1, based upon the results of a study conducted under a US Air Force Avionics Integrity Program, temperature was identified as a causal factor in 55% of electronic failures [1]. It might be noted that in most commercial applications, electronic packages are not subjected to nearly as severe an environment in terms of vibration, dust or humidity as military avionics, so the percentage of failures caused by temperature are likely to occupy a larger “piece of the pie.” In addition to the effect of temperature on electronic device reliability, it can also play an important role on CMOS circuit performance. Consequently, it is necessary to provide satisfactory cooling for electronic packages by design and not by accident.

The Challenges of Electronic Cooling: Past, Current and Future

This paper represents my personal recapitualation of my 4 decades of continuous involvement in all phases of electronic cooling, from conceptual design, through engineering development to product implementation. The cooling designs that we applied successfully in the past are reviewed chronologically. The challenges we are currently facing are also discussed and an attempt is made to forecast the challenges that will confront the electronics cooling community in the near and distant future. The paper includes a summary of IBM sponsored research spanning a period of 25 years at 12 universities on a wide range of topics related to electronic cooling technology.Copyright